Support for `dataclass`es

Given the close similarity between msgspec.Struct and dataclasses.dataclass, am curious to what extent dataclasses can be used with msgspec and what this entails.

[jcrist]

Thanks for the ping (and the nice benchmark plot). I’ve actually been revisiting this opinion, and I think you’ve convinced me that adding simple support for dataclasses is worth it. Now that we have support for TypedDict and NamedTuple objects, asking for dataclass support isn’t that far off. And compatibility with orjson is a convincing use case. Users really should use msgspec.Struct objects instead when possible (they’re much faster, and have fewer weird edge cases than dataclasses), but we can do a decent job with encoding/decoding dataclasses too.

I spent some time this evening experimenting with an implementation, and I’m pretty happy with the results. Encoding time is already much faster than what orjson provides, especially when slots=True is set:

In [1]: import msgspec, orjson

In [2]: from dataclasses import dataclass

In [3]: enc = msgspec.json.Encoder()

In [4]: @dataclass
   ...: class NoSlots:
   ...:     field_one: int
   ...:     field_two: int
   ...: 

In [5]: @dataclass(slots=True)
   ...: class Slots:
   ...:     field_one: int
   ...:     field_two: int
   ...: 

In [6]: no_slots = [NoSlots(i - 1, i + 1) for i in range(10000)]

In [7]: with_slots = [Slots(i - 1, i + 1) for i in range(10000)]

In [8]: %timeit enc.encode(no_slots)  # msgspec, no slots
561 µs ± 2.04 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

In [9]: %timeit orjson.dumps(no_slots)  # orjson, no slots
834 µs ± 1.69 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

In [10]: %timeit enc.encode(with_slots)  # msgspec, with slots
779 µs ± 20 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

In [11]: %timeit orjson.dumps(with_slots)  # orjson, with slots
3.71 ms ± 90.1 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

In [12]: class Struct(msgspec.Struct):
    ...:     field_one: int
    ...:     field_two: int
    ...: 

In [13]: structs = [Struct(i - 1, i + 1) for i in range(10000)]

In [14]: %timeit enc.encode(structs)  # msgspec structs
356 µs ± 307 ns per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

I have JSON encoding done already. Decoding will take a bit more work, but I’d estimate done in the next week or two. I’ll reopen this for now as a reminder.

[jcrist]

Ok, #218 is in - msgspec now supports encoding/decoding dataclasses. This came together a lot quicker than I expected, and was pretty fun to work on. They’re not as performant or featureful as Struct types, but perforamance isn’t bad (especially for encoding).

I have a few other small fixups I’d like to get in, I’d expect a release sometime next week.